The present invention relates to stroke mode or vector driven CRT displays, and more particularly to an apparatus for correcting for the inherent lag between the X and Y CRT channel signals and the video or Z axis channel signal.
Stroke mode CRT displays, also known as vector driven displays, are a well-known type of CRT display. In contrast to raster displays, wherein the beam is driven through a predetermined set of lines according to a particular sweep rate and refresh rate to provide substantially complete beam coverage for a given frame, and the beam turned on at particular pixels to create a particular image, stroke mode CRT displays do not employ a predetermined, repetitive beam line format, but rather the X and Y beam deflection amplifiers are independently driven or controlled so as to draw a particular line or symbol.
Symbol video data written in stroke mode on a CRT has to be corrected at the beginning and end of each stroke, because magnetic deflection amplifier and deflection coil circuitry are necessarily slow at startup and stopping, due to the delayed response characteristics of the beam deflection circuitry. At startup, the beam may take typically 300 nanoseconds to start moving, and a similar time interval to stop moving at the end of a stroke. The X and Y coil drive current waveforms thus typically lag the respective amplifier input by 300 nanoseconds. This is illustrated in FIG. 1A, which depicts the control signal e, and the resulting deflection amplifier drive current i.sub.L passing through the deflection coil.
One solution to this problem has been to stretch the video or Z axis beam signal to correct for the stop delay. FIG. 1B illustrates the video signal in time relation to the deflection amplifier signals of FIG. 1A. As shown in FIG. 1B, the video signal is turned on before the beam begins to move (i.e., deflect), and is turned off before it reaches its end point. The conventional compensation has been to stretch the video until it reaches its endpoint (FIG. 1C), e.g., by use of a one-shot device. However, with this correction, the start and the end of a stroke are overbright because the beam is on before the beam gets up to speed, and the beam remains on for a short time after the beam stops moving, causing bright spots.